thank God, my husband allowed me to become orthodox.
if not (as i was already married), i would have been secretly orthodox and taken my orthodox study Bible allong to the protestant church to flick through during the sermon.
but without going into detail on my side either, i can understand the stress when you have become very close to someone, sharing all that was important to you at the time, and then for one reason or another you find there are very important things that you can't agree on.
can i suggest you spend time (i'm sure u already did, but i mean more time) fasting and praying to God for guidance?
He does have a plan for your life and He does care for your girlfriend as well. so pray and check out first whether you are both trying to serve God in the relationship. like do you care for the other person first, do you serve God before your own interests or hobbies, do you want to serve God by helping the poor, sharing the gospel and promoting justice and peace ahead of your own personal comfort?
these are all basic questions which, if answered honestly can help bring you closer to God.
this is the sort of thing that is meant by 'seek first the kingdom of God'.
it may be that it is right for you to get married outside the church and then you become orthodox later and spend (possibly) many years waiting to share what's really important to you with your wife. (i am deliberately making it sound difficult so you don't do it lightly)
i am assuming you are not baptised yet, if you are, then you can't actually marry someone who does not want to join the orthodox church.
in which case, you still need to answer the questions above, but you also need to make sure your girlfriend understands the situation and it's seriousness.
is she a Christian? can you pray about this situation together?
i think the best way is to be honest and open and ask other people who know you to also pray if possible.
if you wish for God's glory above all else, He will guide you and show you what is best for your soul and for this lady's soul too.
may God guide you and give you peace, and pray for me too, a sinner.